inside out

inside out idiom
  1. With the inner surface turned out or revealed, as in He wore his shirt inside out. This expression dates from about 1600 and was soon used figuratively, as in He turned the verses inside out and revealed their hidden sense.
  2. Extremely well, thoroughly, especially alluding to knowing something. For example, He knows this system inside out. [First half of 1900s]

The American Heritage® Dictionary of Idioms by Christine Ammer. Copyright © 2003, 1997 by The Christine Ammer 1992 Trust. Published by Houghton Mifflin Harcourt Publishing Company. All rights reserved.

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